High-efficiency, multilevel, diffractive optical elements

ABSTRACT

A high-efficiency, diffractive optical element having at least one surface including multilevel steps, the steps determined by calculating equiphase boundaries utilizing a disclosed equation and algorithm. The optical element can be adapted to correct for chromatic and/or spherical aberration, and can be used in UV lithographic apparatus.

The Government has rights in this invention pursuant to Contract NumberF19628-85-C-0002 awarded by the Department of the Air Force.

This is a divisional of co-pending application Ser. No. 099,307 filed onSep. 21, 1987 now U.S. Pat. No. 4,895,790.

BACKGROUND OF THE INVENTION

This invention relates to high-efficiency, on-axis, multilevel,diffractive optical elements. The high efficiency of these elementsallows planar or spherical elements to be diffractively converted togeneralized aspheres, and dispersive materials can be diffractivelycompensated to behave as achromatic materials over broad wavebands. Thetechnique cf this disclosure allows ready implementation of this mixedreflective, refractive and diffractive optics in real systems.

The ability to produce arbitrary phase profiles allows for an additionaldegree of freedom in designing optical systems. Many optical systems nowincorporate aspheric refractive surfaces to produce such phase profiles.System design is restricted by constraints imposed by factors such ascost, size, and allowable asphericity. Diffractive elements arepotentially as versatile and useful as aspheric surfaces and are lessexpensive, and not as subject to asphericity constraints. Anotherobjective in designing optical systems is to minimize chromaticaberrations. Refractive optical materials are chromatically dispersive.Conventionally, the approach to minimizing chromatic aberrations is tobalance the dispersive effects of two different refractive materials.Diffractive surfaces are also wavelength dispersive. It is thereforepossible to take a dispersive refractive element, and by placing adiffractive profile on one of its surfaces, produce an element thatbalances the chromatic effects of the refractive element against thechromatic effects of the diffractive surface. Computer generateddiffractive elements have been proposed for numerous applications suchas chromatic correction, aberration compensated scanners, and hiqhnumerical aperture lenses. A major obstacle to implementing on-axisdiffractive elements in actual systems is the, up to now, lowdiffraction efficiency (<50%).

Theoretically, on-axis diffractive phase elements can achieve 100%diffraction efficiency. To achieve this efficiency, however, acontinuous phase profile is necessary (See, Miyamotc, K., 1961, JOSA 51,17 and Lesem, L., Hirsch, P., Jordan, J., 1969, IBM J. Res. Dev 13,150.) The technology for producing hiqh-quality, hiqh-efficiency,continuous phase profiles does not exist. It has been suggested toquantize the continuous phase profile into discrete phase levels as anapproximation to the continuous phase profile. (Goodman, J., Silvestri,A., 1970, IBM J. Res. Dev. 14, 478.) It is known to make such structuresusing thin-film deposition techniques and material cutting technology(See, U K. Patent Application No. 8327520 entitled "Bifocal ContactLenses Having Diffractive Power".) L. d'Auria et al. in"Photolithographic Fabrication of Thin Film Lenses", OPTICSCOMMUNICATIONS, Volume 5, Number 4, July, 1972 discloses a multilevelstructure involving successive maskings and etchings of a silicondioxide layer. Each mask gives only one additional level in thestructure and is therefore inefficient. The invention disclosed hereinis a method for accurately and reliably making multilevel diffractivesurfaces with diffraction efficiencies that can be as hiqh as 99%.

SUMMARY OF THE INVENTION

In one aspect, the invention is a high-efficiency, diffractive opticalelement having at least one surface including multilevel steps, thesteps determined by calculating equiphase boundaries utilizing theequation ##EQU1## and the algorithm

    ______________________________________                                                  Equiphase Boundaries                                                Level #N  (l = 0, ±1, ±2, . . .)                                                                 Phase Etch Depth θ                             ______________________________________                                        1         φ(x, y) = (l + 1)π                                                                    π                                                            ##STR1##                                                                                     ##STR2##                                            3                                                                                        ##STR3##                                                                                     ##STR4##                                            4                                                                                        ##STR5##                                                                                     ##STR6##                                            ______________________________________                                    

The optical element may be adapted to correct for chromatic aberrationand/or spherical aberration.

In another aspect, the invention is an optical system in a UVlithographic apparatus, the optical system including at least oneoptical element with a multilevel step pattern adapted for aberrationcorrection on one surface.

In yet another aspect, the invention is a UV lithographic exposuresystem including a source of UV radiation and an optics column forgenerating an optical beam or image. The optics column includes at leastone optical element with a multilevel step pattern adapted foraberration correction on one surface. In one embodiment, the opticalelements are adapted to correct for both spherical and chromaticaberration. In a preferred embodiment the steps of the multilevel steppattern are determined by calculating equiphase boundaries utilizing theabove equation and algorithm. In embodiments, the UV radiation has awavelength in the deep UV, and the source of the UV radiation is aneximer laser or a mercury lamp.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a, 1b, and 1c are schematic illustrations of Fresnel phase zoneplate profiles;

FIG. 2 is a graph of first order diffraction efficiency in a multilevelzone plate as a function of the number of phase levels and fabricationmasks;

FIG. 3 is a schematic representation of a binary element fabricationtechnique disclosed herein;

FIG. 4 is a drawing made from a scanning electron microscopephotomicrograph of an eight-level Fresnel zone plate made in accordancewith the present invention;

FIGS. 5a and 5b are drawings of diffraction patterns showing sphericalaberration in an uncorrected and corrected quartz lens, respectively;

FIG. 6 is a graph showing diffractive and refractive dispersion;

FIGS. 7a, b and c are point spread function plots showing diffractivecorrection of silicon lenses;

FIG. 8 is a drawing of the diffractively corrected silicon lens; and

FIG. 9 is a schematic illustration of a UV lithographic exposure systemutilizing the multilevel structures of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1a shows an example of a Fresnel zone plate having a continuousphase profile capable of achieving 100% efficiency. The 2π phase depthcorresponds to a material depth of about one micrometer for visiblelight. Because the technology to produce the continuous phase profile ofFIG. 1a does not exist, an approximation to the continuous phase isdesirable. FIGS. 1b and 1c show Fresnel phase zone plate profilesquantized to two and four phase levels, respectively. The two levelphase profile of FIG. 1b results in a diffraction efficiency of 40.5%,and the four-level profile of FIG. 1c results in an efficiency of 81%.For certain optical applications, such discrete phase structures need toachieve a diffraction efficiency of 95% or higher. FIG. 2 shows thediffraction efficiency as a function of the number of discrete phaselevels. Eight phase levels achieve 95% efficiency.

The method of the invention accurately and reliably produces multilevelon axis, diffractive optical surfaces. Optical elements can be made foruse at wavelengths ranging from the ultraviolet to the infrared. Thesemultilevel structures are useful not only for monochromatic light, butalso for systems operating with fractional bandwidths as large as 40%.The methods disclosed herein take advantage of technology developed forelectronic circuit fabrication such as high resolution lithography, maskaligning, and reactive ion etching. The process for defining the phaseprofile to be constructed will now be discussed.

Collimated monochromatic light incident on a phase Fresnel zone plate(FIG. 1a) will be diffracted with the light being focused perfectly. Thenecessary phase profile can be expressed in the simple form ##EQU2##where λ is the wavelength, F the focal length, and φ is evaluted modulo2π. The phase Fresnel zone plate is an interesting yet limited exampleof a profile. In general, it is desirable to define arbitrarydiffractive phase profiles.

There exist numerous commercially available lens design programs. Manyof these programs allow one to describe a general diffractive phaseprofile on a given surface. The phase profile is described by making ananalogy to the optical recording of holographic optical elements. Thewavelength and location in space of two coherent point sources aredefined and the resulting interference pattern describes the diffractivephase profile. This process describes more general profiles than asimple zone plate, however, which is still a small subset of thepossible profiles. In order to make the phase profiles span a muchlarger set of possibilities, an additional phase term ##EQU3## can beadded onto the phase determined from the two point sources. For on-axisphase profiles, the two point sources must lie on the optical axis.Furthermore, if the locations of the two point sources are both set toinfinity, then the effect of their interference is null and the phaseprofile is completely described by the general polynomial expansion ofequation (2). One of these general diffractive phase profiles cantherefore be placed on any surface of an optical system.

Lens design programs have optimization routines that treat thecurvatures of surfaces, the thickness of elements, and the elementspacings as variables. Likewise, if a diffractive phase profile is inthe system, the optimization routine can treat the polynomialcoefficients, a_(nm), as variables. A lens optimization program willdetermine the optimum coefficients, a_(nm), of the diffractive phaseprofile for any particular lens systems.

The diffractive phase profile determined by the lens design program anddefined by equation (2) contains no information on how to achieve highdiffraction efficiency. Our approach is to take the optimized a_(nm) 'sand from them define a set of binary amplitude masks. The algorithm fordesigning these masks is shown in Table 1.

The equation φ(x,y)=C, where C is a constant, describes an equiphasecontour. Mask 1 describes the set of equiphase contours that are integermultiple of π. Mask (n=2,3, . . . ) describes the set of equiphasecontours that are integer multiples of π/(2.sup.(n-1)).

The area between the first two sequential equiphase boundaries islithographically exposed. The areas between subsequent sequentialequiphase boundaries alternate from not being exposed to being exposed.This process is repeated until the total pattern is drawn, covering thefull optical aperture.

Table 1 also indicates the phase depth θ to which various lithographicmask patterns are etched The relationship between phase depth andmaterials depth d is simply ##EQU4## where n is the refractive index ofthe optical material. Column 4 of Table 1 indicates the relationshipbetween the number of phase levels k=2^(N) and the number of masks N.

Column 5 indicates the achievable diffraction efficiency η. It isremarkable and an important point of this disclosure that with a merefour masks, 99% diffraction efficiency can be achieved. These binaryamplitude masks will then be used in the actual construction of a highlyefficient diffractive phase profile.

                  TABLE 1                                                         ______________________________________                                        Multimask Design Algorithm                                                     ##STR7##                                                                                           Phase                                                          Equi-phase Boundaries                                                                        Etch     # Phase                                        Mask #N                                                                              (l = 0, ±1, ±2, . . .)                                                                 Depth θ                                                                          Levels κ                                                                       % eff. η                            ______________________________________                                        1      φ(x, y) = (l + 1)π                                                                    π     2      40.5                                            ##STR8##                                                                                     ##STR9##                                                                              4      81.0                                    3                                                                                     ##STR10##                                                                                    ##STR11##                                                                             8      95.0                                    4                                                                                     ##STR12##                                                                                    ##STR13##                                                                             16     99.0                                    ______________________________________                                    

Three tools necessary for practicing the method of the present inventionhave been developed over the past ten years by the semiconductorindustry. They include sub-micron lithography, ion etchers, and maskaligners. Lithographic pattern generators are capable of drawing binaryamplitude masks with feature sizes of 0.1 μm and positioning thefeatures to an even greater accuracy. Reactive ion etchers can etch abinary profile to depths of a few microns with an accuracy on the orderof tens of angstroms. Mask aligners are used routinely to align twopatterns with an accuracy of fractions of a micron. These are the keytechnological advances that make it possible to produce high qualitydiffractive phase profiles.

Electron beam pattern generators produce masks that have binarytransmittance profiles. A thin layer of chromium on an optically flatquartz substrate is patterned by e-beam lithography. The input to thee-beam pattern generator is a file stored on a computer tape andproperly formatted for the particular machine. For multileveldiffractive elements, the algorithm described in Table 1 defines thepatterns to be drawn The number of phase levels in the final diffractiveelement constructed from these masks is 2^(N), where N is the number ofmasks. For example, only four masks will produce 16 phase levelsresulting in an efficiency of 99%.

The binary amplitude masks produced from the pattern generator are thenused in a serial fashion to construct the multilevel optical element.The fabrication process using the first mask is shown in FIG. 3. Anoptical substrate 10 such as SiO₂ on which the diffractive profile is toreside is coated with a layer of chromium 12 and a layer of photoresist14. An e-beam generated mask 16 is then placed over the substrate 10 andilluminated with a standard uv photoresist exposure system (not shown).The photoresist layer 14 is then developed resulting in a properlypatterned layer cf photoresist. The photoresist acts as an etch stop forthe reactive ion etching.

Reactive ion etching (RIE) is a process in which an RF electric fieldexcites a gas to produce ions. The ions react with the material of thesubstrate and etch away the surface at a controlled rate. The reactiveion etching process is anisotropic so that the vertical side walls ofthe discrete phase profile are retained. Typical RIE etch rates are onthe order of 100 Angstroms to 200 Angstroms per minute. As an example,the required first level etch depth for a quartz substrate to be used ata wavelength of 6328 Angstroms is 7030 Angstroms. The necessary etchtime is on the order of one-half hour and numerous elements can beetched simultaneously. After the pattern of the first mask has beenetched into the substrate, any residual photoresist and chromium arestripped away.

The same procedure outlined above is then repeated on the opticalsubstrate 10, only this time using a second mask and etching to one halfthe depth of the first etch. For the second and subsequent masks anadditional complication arises. These masks have to be accuratelyaligned to the already existing pattern produced from an earlier etch.Fortunately, the problem of accurately aligning patterns has been solvedby the integrated circuit industry. Commercially available mask alignersare capable of aligning two patterns to a fraction of a micron. Thisaccuracy is sufficient to retain diffraction limited performance for themajority of the multilevel structures designed to operate in the visibleand infrared.

The simplest example of a diffractive optical element is the Fresnelzone plate described by equation (1). The applicants herein have carriedout the above procedure and produced, with three masks, an eight levelFresnel zone plate. FIG. 4 is a drawing made from an SEM photograph ofthe element. The element was designed for use with a HeNe laser ofwavelength 6328 Angstroms and is a quartz substrate with a diameter oftwo inches. The experimentally measured diffraction efficiency of theelement was 92%. Other multilevel phase Fresnel zone plates have beenmade for use with GaAs laser diodes.

In addition to Fresnel zone plates, the methods of the invention areutilized in making refractive/diffractive combination optical elements.Fresnel zone plates are, in practice, useful for collimating amonochromatic point source of light. An aspheric conventional lens canperform the same function at considerably higher cost. A spherical lensis significantly less expensive yet cannot achieve perfect collimation.It is, however, possible to take a spherical lens and calculate from alens design program the necessary diffractive profile that when etchedinto a surface of the spherical lens will result in perfect collimation.

FIGS. 5a and 5b illustrate aberration correction utilizing the opticalelements according to the present invention. FIG. 5a shows anuncorrected spherical aberration pattern produced by a quartz lens whentested with a HeNe laser at 6328 Angstroms. Note that FIG. 5a shows a150 micron wide point-spread function exhibiting classical sphericalaberration. FIG. 5b shows the results when the lens includes aneight-phase-level pattern etched into the back surface of aplano-spherically convex quartz lens. The eight-phase-level pattern madeusing three masks in effect turns a spherical lens into anear-diffraction-limited asphere. Note that the power of the six micronfocal point shown in FIG. 5b is increased nearly two hundred-fold over asimilar spot in FIG. 5a. Such an optical element will have bothrefractive and diffractive properties.

The disclosed technique can not only correct for spherical aberrationsin imperfect optics but for chromatic aberrations as well. All opticalmaterials are dispersive. Dispersion is an undesirable property thatmust be avoided in broadband optical systems. Generally this is done bybalancing the dispersive property of two different optical materials. Anachromatic lens is therefore usually a doublet or a triplet lens. Thisapproach leads to expensive and bulky optics. With efficient diffractiveoptics as disclosed in this patent application chromatic balancing withmultiple elements can be avoided altogether. The diffractive focal powerof a combined diffractive-refractive lens can be used to balance thechromatic dispersion of the conventional lens provided the ratio of thediffractive to refractive focal lengths at the center wavelength is##EQU5## In Equation 3, n_(c) is the index of refraction of theconventional material at the center wavelength, λ_(c), and d is thedispersion constant of the material, i.e., the slope of the index ofrefraction vs. wavelength curve.

FIG. 6 shows this concept. The compensating dispersion is linearlyproportional to the focal length of the diffractive component. Curve 20represents the dispersion due to the bulk dielectric of the conventionallens and curve 22 to the dispersion of the diffractive component. Thehorizontal axis represents the wavelength bandwidth over which thecompensation occurs and the vertical axis represents the optical power(1/F). Adding the optical powers of the refractive and diffractivecomponents together results in curve 24. By satisfying Equation 3, theoptical power (and therefore focal length) can be made constant over thewavelength band.

Balancing of the chromatic aberration can occur over a very largebandwidth. Its width clearly depends on the used wavelength, thesystem's application, and on the linearity of the chromaticity of therefractive lens component.

FIGS. 7a, b and c show a design comparison of an F/2 silicon lens in the3-5 micron waveband. FIG. 7a shows the point spread function of aconventional spherical lens. FIG. 7b shows the point spread function ofa conventional aspheric lens and FIG. 7c shows the diffraction limitedoperation when both spherical and chromatic aberration corrections areetched into the surface of a simple spherical lens. FIG. 8 shows acorrected silicon lens made by the multilevel process.

A particularly useful embodiment of the present invention is insemiconductor UV lithographic systems where a lack of good transmissivematerials (UV grade silica is one of a few) makes conventional broadbandchromatic correction nearly impossible. Even microlithography systemsbased on KrF eximer lasers are severly limited by the lack of suitableUV transmitting and achromatic materials. At or below 2500 Angstroms,even fused silica is so dispersive that a few Angstroms bandwidthimposes intolerable chromatic and spherical aberrations. The multilevelstructures of the present invention will improve-dramatically thecapabilities of equipment such as contact printers, projection andproximity wafer printers, step-and-repeaters, microscopes, mask patterngenerators, and mask aligners, all of which are based on UV mercury lampor UV eximer laser optics. The binary corrective patterns for UVlithographic lenses have periodicities and feature sizes that are farlarger than the UV wavelength used. A typical projection printer lensmay have minimum features in the needed binary pattern of 2-5 micronsThus, it is feasible to fabricate UV binary lenses, taking intoconsideration materials and pattern resolution constraints.

A shift from λ=3500 Angstroms to λ=1900 Angstroms can double circuitdensity. Present efforts with KrF eximer laser technology are limited to10⁻⁴ fractional bandwidths. With binary optics chromatic corrections thelimits can be extended to 10⁻². Therefore, the throughput can increaseby a factor of 100 with additional benefits of reduced sensitivity toimage speckle and dust. Another less obvious benefit of the reducedwavelength is a doubling of depth of focus. This doubling relaxesmechanical alignment tolerances in proximity printers and extends masklifetimes.

With the technique described in this disclosure a

1) one hundred-fold increase in throughput of lithographically patternedcircuitry may be possible;

2) shift into deep UV may increase circuit density by a factor of two;and

3) shift to deep UV will also relax proximity restraints in submicroncircuit designs by increasing the depth of focus by as much as 75%.Semiconductor International. May 1987, page 49

All this is possible because of fundamental dielectric materialsconstraints in purely refractive optical systems are eliminated orrelaxed by the diffractive techniques described in this disclosure. Thetechniques according to the invention can thus be used for etchingdiffractive profiles into a lens surface to effect chromatic andspherical aberration correction for UV lithographic systems.

FIG. 9 shows a lithographic exposure system to reach deep UV forresolving 0.25 micron features. An eximer laser or mercury lamp source30 illuminates an optics column 32 including on the order of 5 or 6optical elements having the multilevel structures of the invention. Theoptics column 32 replaces conventional optics columns known in prior artlithographic exposure systems. Such conventional columns include manymore optical elements than the column 32.

It should be noted that, as in circuit fabrication process, one set ofmasks can be used repeatedly to produce a large number of diffractiveoptical elements. Also, these diffractive surface profiles can be copiedin metal using electroplating techniques. The metal master can then beused to emboss in plastic a large number of replicated opticalcomponents. The metal mastering and embossing replication is anestablished art.

What is claimed is:
 1. High-efficiency, diffractive optical elementhaving at least one surface including multilevel steps, the stepsdetermined by calculating equiphase boundaries utilizing the equation##EQU6## where x and y denote coordinates on the optical element, λdenotes wavelength, and a_(nm) represents a series of optimizedcoefficients, and the algorithm

    ______________________________________                                                  Equiphase Boundaries                                                Level #N  (l = 0, ±1, ±2, . . .)                                                                 Phase Etch Depth θ                             ______________________________________                                        1         φ(x, y) = (l + 1)π                                                                    π                                                            ##STR14##                                                                                    ##STR15##                                           3                                                                                        ##STR16##                                                                                    ##STR17##                                           4                                                                                        ##STR18##                                                                                    ##STR19##                                           ______________________________________                                    


2. The high-efficiency, diffractive optical element of claim 1, whereinthe multilevel steps are adapted to correct for chromatic aberration. 3.The high-efficiency, diffractive optical element of claim 1 wherein themultilevel steps are adapted to correct for spherical aberration.
 4. InUV lithographic apparatus, an optical system including at least oneoptical element, one surface of said optical element including amultilevel step pattern adapted for aberration correction.
 5. UVlithographic exposure system comprising:a source of UV radiation; and anoptics column for generating an optical beam or image, the optics columnincluding at least one optical element, one surface of which includes amultilevel step pattern adapted for aberration correction.
 6. Thelithographic exposure system of claim 5 wherein the at least one opticalelement corrects both for spherical and chromatic aberration.
 7. Thelithographic exposure system of claim 5 wherein the steps of themultilevel step pattern are determined by calculating equiphaseboundaries utilizing the equation ##EQU7## where x and y denotecoordinates on each optical element, λ denotes wavelengths, and a_(nm)represents a series of optimized coefficients, and the algorithm

    ______________________________________                                                  Equiphase Boundaries                                                Level #N  (l = 0, ±1, ±2, . . .)                                                                 Phase Etch Depth θ                             ______________________________________                                        1         φ(x, y) = (l + 1)π                                                                    π                                                            ##STR20##                                                                                    ##STR21##                                           3                                                                                        ##STR22##                                                                                    ##STR23##                                           4                                                                                        ##STR24##                                                                                    ##STR25##                                           ______________________________________                                    


8. The lithographic exposure system of claim 5 wherein the source of UVradiation has a wavelength in the range of substantially 190 nm to 300nm.
 9. The lithographic exposure system of claim 5 wherein the source ofUV radiation is an eximer laser.
 10. The lithographic exposure system ofclaim 5 wherein the UV radiation source is a mercury lamp.